The present invention relates to prereforming of natural gas. More specifically, the invention relates to a novel use of a catalyst in the step of adiabatic steam-reforming to improve the production of hydrogen and syngas.
The steam reforming process is routinely used in the chemical processing industry to produce hydrogen or a synthesis gas comprising a mixture of hydrogen and carbon monoxide, from natural gas. The reforming process is generally carried out at a high temperature and pressure to facilitate reaction between the steam and a hydrocarbon feedstock in the presence of a nickel catalyst supported on alumina or another suitable material.
Several advancements have been made in recent years to improve the overall process economics of steam reforming. A significant amount of research has focused on developing coke resistant nickel-based catalysts. The development of coke resistant catalysts was beneficial because of the presence of higher hydrocarbons in the natural gas that were known to deactivate conventional reforming catalysts by coke formation.
Another advancement is the adoption of adiabatic prereforming, which traditionally has as its primary purpose the conversion of feedstocks that are difficult to steam reform in a fired, tubular reformer (e.g., butane, naphtha) into prereformed feeds that are easier to reform. Therefore, a number of nickel-based prereforming catalysts were developed specifically for treating heavier feedstocks.
More recently, some companies have used prereforming for a very different purpose, namely, to reduce the quantity of byproduct steam that is produced along with the primary hydrogen or syngas product. Prereformers can achieve this objective by allowing waste heat in the flue gas to be used for preheating, prereforming and reheating of the feed rather than for just preheating and steam generation. Standard prereforming catalysts, which were developed for the processing of heavier feedstocks, have been used in prereformers even if their role has shifted to that of reducing steam production with a natural gas feed.
In summary, a prereformer properly integrated with the main reformer can offer a number of benefits for prereforming natural gas including: (1) reducing the amount of byproduct steam, (2) reducing the load on the main reformer by converting a part of methane present in the feed stream, (3) reducing the possibility of coke formation on the main reformer catalyst by converting most of the higher hydrocarbons present in the feed stream, (4) reducing the ratio of steam to natural gas required for the reforming reaction, (5) providing flexibility in processing the natural gas feed from different sources, (6) providing the luxury of preheating the gaseous feed mixture to a higher temperature prior to introducing it into the main reformer, and (7) increasing the life of the catalyst and tubes in the main reformer.
Limited research has focused on the development of catalysts for prereforming natural gas. As mentioned before, conventional prereforming catalysts that were developed for treating heavier feedstocks are still used for prereforming natural gas. The conventional prereforming catalysts are microporous, have a high surface area, and contain high nickel content. They are temperature sensitive; exposure to excessive temperatures will cause sintering, carbon formation, and loss of activity. As a result, the feed gas temperature is limited to less than 550° C. because the catalyst deactivates rapidly above this temperature. The conventional catalysts are also sensitive to steaming, and therefore special procedures are required to bypass the prereformers during startup and shutdown. In addition, the catalysts require change-out every two to three years. For example, see U.S. Pat. No. 4,105,591; GB 969,637; GB 1,150,066; GB 1,155,843; U.S. Pat. No. 3,882,636; and U.S. Pat. No. 3,988,425.
Another type of nickel catalyst has been used inside fired tubular reformers for several years. This type of catalyst is exposed to temperatures considerably higher than that used in conventional prereformers. This type of catalyst contains a lower amount of nickel than conventional prereforming catalysts and is supported on calcium aluminate. Based on commercial experience, this low nickel containing catalyst deactivates much slower than commercial prereforming catalysts. This characteristic is due to the catalyst's superior resistance to sintering and breakage. However, the industry has traditionally thought of such catalysts as inappropriate for use in adiabatic prereformers because they would not have the required activity.
Despite this traditional thinking, use of a low nickel containing catalyst in an adiabatic prereforming process of light natural gas is disclosed in EP 1241130A1. The process comprises an inlet temperature of 500 to 750° C., using a catalyst containing 3 to 20% nickel on aluminum oxide or calcium aluminate support with a specific bimodal pore structure: greater than 8% porosity for 5,000 to 200,000 A pores and greater than 15% porosity for pores less than 5000 A, with a total porosity between 23% and 80%. This patent does not claim the use of a low nickel catalyst that has been promoted with an alkaline material such as potassium, and in fact it discourages the use of alkali promotion with a statement implying that alkali or potassium in the catalyst would reduce activity: “ . . . a nickel catalyst containing an alkali component is used in a portion or all of the reformer tubes in the heating furnace. Since this catalyst improves carbon-depositing resistance at the sacrifice of activity, it has a disadvantage that a necessary amount of the catalyst is large.”
U.S. Pat. Nos. 4,990,481 and 5,773,589 also use low nickel containing catalysts, but disclose only steam reforming under isothermal conditions, not adiabatic prereforming. Additionally, the catalyst in U.S. Pat. No. 4,990,481 is not promoted with an alkaline material, and neither patent discloses any benefit that would result from doing so.
Although the use of prereformers and the use of commercially available prereforming catalysts have been very effective in preventing higher hydrocarbons from entering the main reformer, it is clear that other problems with the process still exist. In particular, the operation of prereformers has been plagued by deactivation of a nickel containing catalyst, most likely due to coke formation, catalyst instability, sintering, oxidation, sulfur poisoning, or some other factors.
Accordingly, it is desired to provide a prereforming process, wherein said process does not substantially suffer from the aforementioned deficiencies of other processes. It is further desired to provide a natural gas prereforming process, wherein the performance and durability of the catalyst is improved, and to develop a catalyst specifically for the prereforming of natural gas.
All references cited herein are incorporated herein by reference in their entireties.